DVB-C2 (Digital Video Broadcasting for Cable 2) serves as a European second-generation cable digital broadcasting standard (Non-Patent Document 1).
FIG. 1 is a drawing depicting an example of a C2 system. The horizontal axis of FIG. 1 represents frequency. One DVB-C2 signal is referred to as a C2 system, and includes a preamble symbol and a data symbol. In accordance with the standard, one C2 system constitutes a signal having a maximum bandwidth of approximately 3.5 GHz.
A preamble symbol is a symbol used for the transmission of L1 information (L1 signalling part 2 data), which is transmission control information. A preamble symbol is used for the same information to be repeatedly transmitted in a 3408 carrier period (OFDM (orthogonal frequency division multiplex) 3408 sub-carrier period).
A data symbol is a symbol used for the transmission of a TS (transport stream) or the like for program data and so on. A data symbol is divided into blocks referred to as data slices. For example, respectively different program data is transmitted in data slice 1 (DS1) and data slice 2 (DS2). Parameters relating to the data slices such as the number of data slices are included in the L1 information.
As depicted by the black filled-in parts in FIG. 1, it is possible for a notch to be included in a C2 system. A notch is a frequency band used for FM broadcasts, police wireless broadcasts, and military wireless broadcasts or the like, and is not used for the transmission of a C2 system. A notch section within a transmission signal output by a transmission device constitutes a non-signal section. With regard to notches, there are narrowband notches that have a bandwidth of 47 carriers or less, and broadband notches that exceed 47 carriers. Parameters relating to notches such as the number and bandwidths of notches are also included in the L1 information.
In this way, in DVB-C2, it is not necessary to provide guard bands between channels and, furthermore, because relatively narrow bands interposed between notches can also be used for the transmission of data, it is possible for frequency bands to be effectively used. Furthermore, it is possible for the allocation of frequency bands for broadcast signals to be carried out in a flexible manner.
However, with the flexible allocation of frequency bands being possible, from the point of view of a reception device, during a channel scan, it is difficult to detect the frequency band used for the transmission of a DVB-C2 OFDM signal.
In other words, in the case where frequency bands are allocated in a fixed manner by a country or a broadcasting organization as depicted in A of FIG. 2, a reception device can detect the frequency bands of broadcast signals if the presence/absence of broadcast signals is determined in accordance with the allocation of known frequency bands.
However, in the case of DVB-C2, sometimes a DVB-C2 OFDM signal constituting a desired signal deviates from the reception signal band when signals of predetermined frequency bands are received as depicted in B of FIG. 2. The reception of a DVB-C2 OFDM signal is, as depicted in B of FIG. 2, carried out in such a way that a signal within a tuning window of a bandwidth of 7.61 MHz is received.
FIG. 3 is a drawing illustrating a system for detecting the frequency band of a DVB-C2 OFDM signal.
As depicted in FIG. 3, an FFT calculation is carried out with respect to a time-domain signal acquired by carrying out orthogonal demodulation or the like on a DVB-C2 OFDM signal, and the absolute value (power (Abs(x) in FIG. 3)) for amplitude is obtained on the basis of a frequency-domain signal obtained by the FFT calculation. Furthermore, an average value for power is obtained by means of filter processing using an averaging filter, and the average value for power is compared with a threshold value c. For example, a section in which the average value for power is lower than the threshold value c is, as depicted in the left side of FIG. 4, detected as a guard band (gap) that is ensured between the frequency band for a signal of a certain standard and the frequency band for a signal of another standard.
A reception device, as indicated by the white arrow in FIG. 4, aligns the ends of a tuning window with guard bands and receives a signal, and carries out detection of the frequency band for a DVB-C2 OFDM signal. In this system, with the assumption that a guard band constituting a non-signal buffer section for preventing interference is provided between each signal, the power of the frequency bands is observed, and both ends of the frequency band of each signal are detected.